1. Field of the Invention
The present invention relates to the field of displays for computer systems and, more specifically, to the field of graphical presentation and drawing systems displaying images in gray-scale.
2. Description of Related Art
A number of methods of presentation of color information to display devices are well known in the art. In general, such display devices may be divided into two categories: rod-green-blue (RGB) devices and NTSC or similar devices. In an RGB device, color information is presented to a display as three separate units of color information; a first unit of information representing the intensity of the red color gun of the display, a second unit representing the intensity of the green color gun of the display and a third unit representing the intensity of the blue color gun of the display.
NTSC devices (and their equivalents under other standards such as PAL) present color information to a display generally by phase-shifting a waveform some predetermined number of degrees from a reference signal. The color display, such as a television set interprets color based on the number of degrees the waveform is out of phase with the reference. Such systems may further control the intensity of the color by controlling the amplitude of the color signal.
In the RGB color system, a display may be controlled by presenting bits of color information to drive digital-to-analog converters which in turn produce three analog color signals which control the red, green and blue guns of a display. Typically, 24 bits of color information may be used; 8 bits representing red, 8 bits representing green and 8 bits representing blue. Using 24 bits of color information allows over 16 million (2.sup.24) colors to be produced.
In a typical computer system employing a color display, a device called a "frame buffer" is utilized. A frame buffer is a memory for storing color information corresponding to each pixel on the display. A frame buffer may store 24 bits of information per pixel and the 24 bits of information may be used to directly control the color display. Such a system is typically termed a direct color system. However, use of a full 24 bit frame buffer requires a large amount of memory space and leads to other processing inefficiencies. As an example of the amount of memory space required, many known displays, such as a display which may be utilized with the Macintosh II, available from Apple Computer, Inc. of Cupertino, Calif., have displays comprising 640.times.480 pixels.
It is known to utilize a frame buffer having less than 24 bits of color information per pixel. Such a system may store for example 1, 2, 4, or 8 bits per pixel for color presentation. The bits of information from the frame buffer are used to address a color lookup table (CLUT). The data outputs of the CLUT are the RGB colors signals or their digital equivalents. The use of the CLUT offers a number of advantages. For example, less memory may be used for a frame buffer and colors on the display may be adjusted by adjusting the data content of the CLUT. So, an image represented using a full 24 bits may be displayed on a system having only 8 bits available for display.
Alternatively, some systems may represent images in gray-scale. Gray-scales are sometimes represented as a luminance values. Luminance values are those that range from minimum luminosity (black) to maximum luminosity (white). It can be appreciated that any value of gray depending on the number of bits used to represent the color may be represented between these minimum and maximum values. In an RGB color display system representing gray-scale images, equal intensities or values of red, green and blue are used to represent the corresponding gray or luminance value. A color system using 24 bits may be used for gray-scale images where the red, green and blue values are equal, providing 256 shades of gray (eight values of red, green and blue or 2.sup.8). In addition, color images may be represented in their corresponding gray-scale using a transformation function.
In one prior art approach as shown on FIG. 1, 24 bit RGB color information 102 may be used as an address input to a color lookup table 101. However, as shown on FIG. 1, use of the full 24 bit RGB color information field would require a color lookup table with an address space of 2.sup.24 entries of four bits each for each index. Such a large table is generally not desirable in a computer system as a large amount of computer main memory is consumed.